Conventionally, with respect to an underground liquid storage tank for a gas station, a valve device for preventing overfill is mounted in a liquid supply sleeve which communicates with the underground liquid storage tank. Such a device can effectively prevent the overfill of liquid into the underground liquid storage tank during the operation of filling liquid into the storage tank from a liquid carrier car.
For example, as disclosed in Japanese laid-open patent publication No. HEI 1-213199, the valve device is installed in the liquid supply sleeve. The valve device is provided with a float for closing a valve when a liquid level reaches a predetermined level and such a float has an upper end thereof pivotally attached to the liquid supply sleeve so as to pivot in an upward or downward direction.
Such a valve device is intended to be inserted into the liquid supply sleeve from an upper opening of the sleeve and to be installed in the sleeve in place. Accordingly, to enable the valve device installing operation, the tiltable float must be compactly accommodated in the valve device.
Namely, if the float is extended outwardly from the valve device, inserting of the valve device into the liquid supply sleeve becomes impossible, as it is hindered by the outwardly extended float.
Accordingly, in the above Japanese laid-open publication, a part of the peripheral surface of the cylindrical valve device is flattened or indented such that the tiltable float can be accommodated in the indented part, and when the inserting and installing of the float in the liquid supply sleeve are completed, the float is extended outwardly from the valve device to detect the rise of the liquid level.
In the above-mentioned conventional valve device, however, since the indented part for accommodating the float is formed by indenting the sleeve of the valve device per se, the liquid supply which flows down in the sleeve of the valve device cannot be smoothly fed into the underground liquid storage tank due to the narrowing or throttling of the sleeve cross-section caused by the indented part of the sleeve.
Therefore, liquid tends to be retained in the upper portion of the indented part and eventually overflows from the upper end of the valve device.
For preventing such an accident, the liquid supply amount must be regulated in relation to the throttling of the indented part of the valve device, whereby the liquid supply operation to the underground liquid storage tank requires considerable time, thus worsening the efficiency of the liquid filling operation.
Accordingly, it is an object of the present invention to provide an overfill preventing device for an underground liquid storage tank which can overcome the above-mentioned defects of the conventional valve device.